The present invention relates generally to power amplifiers, and more specifically to switch mode power amplifiers.
Conventional series-regulated, linear power amplifiers typically maintain a constant voltage by varying their resistance to cope with input voltage changes or load current demand changes. These power amplifiers are used in, for example, cellular telephone base stations. Cellular telephone base stations typically consume a large amount of electricity in the process of amplifying signals sent to mobile handsets. As a result, a dominant fraction of the power that is input into the base station is typically dissipated as heat.
To deal with this heat dissipation, cooling fans and/or heat sinks are used so that the electrical components do not overheat. The cooling components often waste valuable space in the circuit cabinet and also consume additional power. For example, more than half of a base station may be occupied by amplifiers and their cooling components.
Switch mode power amplifiers, however, are typically more efficient than other types of power amplifiers. The switch mode power amplifier uses a high frequency switch (e.g., a transistor) with varying duty cycle to maintain an output voltage. The output voltage variations caused by the switching are often filtered out by an inductor-capacitor (LC) filter. Switch mode power amplifiers may operate in a Class-S (supply modulated) mode, which is a combination of a bandpass delta sigma modulator with a Class-D switching amplifier. Other modulators providing the drive switching signal for Class-D may also be implemented, such as a pulse length modulator.
The circuit topologies known to realize switch mode amplifiers are often used in audio applications (i.e., the switch mode amplifiers operate in the kHz frequency range). Recently, however, they have also been used in the microwave range (1-2 GHz), but the signals typically only produce a small amount of power (e.g., 2 W peak). Base stations, however, often need output signals having a peak power in the 100 W range. In the context of base stations, switched mode power amplifiers (e.g., class-S switched mode power amplifiers) are attractive because their high efficiency may enable the reduction of or elimination of cooling components (e.g., cooling fans and/or heat fins) and may lead to lower operational expenditure costs with the base station's operators. Thus, both capital expenditure (CAPEX) savings are present from less cooling effort and operational expenditure (OPEX) savings are present from less energy consumption.
Therefore, there remains a need to have a switch mode power amplifier that can be used in the microwave frequency range and also produce a power output that is an order of magnitude higher than is currently possible.